Aluminium substrates are widely used in automobile parts, such as aluminum wheels. A metallic aluminum wheel can be obtained by coating an optionally treated aluminum substrate with a base coating composition multiple times, and then further coating the substrate with a metallic coating composition containing a vapor-deposition aluminum pigment.
For example, Patent Literature 1 discloses a metallic coating composition comprising an effect pigment obtained by crushing a vapor-deposition metal film into metal pieces, and a phosphate group-containing compound. Patent Literature 1 also discloses a method for forming a metallic coating film comprising forming an undercoating layer on a substrate, applying the metallic coating composition to form a metallic coating film layer, and further forming a clear coating film on the metallic coating film layer. However, simply increasing the aluminum pigment content in view of the disclosure of Patent Literature 1 may impair the coating composition stability or adhesion.
Patent Literature 2 discloses a metallic coating composition comprising a binder resin and metal flakes, wherein the binder resin is present in an amount of 50 to 700 parts by mass per 100 parts by mass of the metal flakes. The metallic coating composition is capable of forming a smooth and flat metallic coating film with excellent appearance because of the well-arranged metal flakes providing excellent metallic appearance, and even forming the top coating layer on the metallic coating film does not disturb the arrangement of the metal flakes. However, the coating composition disclosed in Patent Literature 2 may be insufficient in adhesion, weatherability, and specular gloss.
Patent Literature 3 discloses a metallic coating composition with a solids content of 1 to 10% obtained by adjusting a coating composition containing an acrylic resin, a melamine resin, an effect material, and a cellulose acetate butyrate resin with an ester-based solvent and/or a ketone-based solvent. However, increasing the effect material content in the coating composition disclosed in Patent Literature 3 to achieve specular gloss may impair at least any of coating composition stability, finished appearance, and adhesion.
Patent Literature 4 discloses a metallic coating composition containing an acrylic resin (a), a curing agent (b), an effect pigment obtained by crushing a vapor-deposition metal film into metal flakes (c), and a solvent (d). However, increasing the effect material content in the coating composition disclosed in Patent Literature 4 to achieve specular gloss may impair any of coating composition stability and adhesion.
Patent Literature 5 discloses a coating composition containing a resin, a vapor-deposition metal foil, and a solvent wherein the vapor-deposition metal foil is present in an amount of 100 parts by weight or more to 900 parts by weight or less, per 100 parts by weight of the resin. However, the coating composition disclosed in Patent Literature 5 may be unsatisfactory in at least any of finished appearance, adhesion, and weatherability.
Against such a background, there has been a demand for a metallic coating composition excellent in coating composition stability and capable of forming a coating film excellent in finished appearance, adhesion, weatherability, and specular gloss.